Transmission method to determine and control the temperature of wafers or thin layers with special application to semiconductors

ABSTRACT

A method of accurately determining the temperature of a thin layer of bandgap material without requiring contact to the layer involves the use of optical radiation through the layer and the detection of optical absorption by the layer. The relationship between the temperature varying bandgap energy and the resulting optical absorption characteristics provides an indication of temperature independent of ambient temperature. Apparatus for performing high quality temperature detection and control is also provided.

This application is a division of application Ser. No. 07/157,196, nowU.S. Pat. No. 4,890,933 filed Feb. 17, 1988.

FIELD OF THE INVENTION

The present invention relates to temperature-process control procedures.More particularly, the invention is applicable to the precise detectionas well as control of the temperature of a semiconductor crystal and isespecially useful in process steps where contact with the crystallinesurface must be avoided. An example of an application of the inventioninvolves the processing of GaAs thin layers, or of wafers prior to thegrowth of epitaxial layers, where the temperature must be accuratelyknown and controlled, and no physical contact to the wafer surface canbe tolerated.

BACKGROUND AND SUMMARY OF THE INVENTION

In the processing of semiconductors, such as GaAs in this example, thereare difficulties in measuring the temperature of wafers or of thinlayers, especially in applications where the temperature has to be knownaccurately, and no physical contacts to the wafer (or thin layer) arepermitted. Two examples of processes where these problems arise are"heat-cleaning" of wafers prior to growing subsequent layers on them byMolecular Beam Epitaxy, and pre-activation "heat cleaning" ofphotocathodes. The latter represents the present application of theinvention.

A device which has been used in an attempt to overcome thesedifficulties is the pyrometer, which utilizes the black-body (or "graybody") radiation of the sample in order to measure its temperature. Thismethod, however, is valid only when the wavelength of the radiation usedis such that its characteristic coefficient of absorption is very largein comparison with the reciprocal of the thickness of the wafer or thethin layer. Such is rarely the case with wafers or thin layers ofsemiconductors such as GaAs, since the long wavelength light (λ>1000nanometers) used in pyrometry is hardly absorbed (if at all) by thesemiconductor whose bandgap energy exceeds that of the light. Only forthick wafers, having temperatures well above room temperature, can thepyrometric method be applied: in these situations, wavelengths of about900 nanometers are employed.

Pyrometers, therefore, when used in applications to GaAs or tosemiconductors of comparable bandgaps, almost always monitor thetemperature of the body on which the semiconductor wafer rests ratherthan the actual temperature of the semiconductor material. In the caseof the photocathode bonded to a glass faceplate, the pyrometer(utilizing radiation above 900 nanometers) absorbs radiation emitted bythe glass faceplate. The cathode itself, which is totally transparent tosuch radiation, is not "seen" at all by the pyrometer; and, furthermore,the cathode layer introduces an additional complication by acting as athin film interference filter. This latter effect causes the pyrometrictemperature readings of the glass faceplate itself to be in error --depending on the thickness of the cathode layers. The thinner thecathode layer, the more sensitive the pyrometer reading to smallvariations in the layers' thickness.

The present invention is based on the monotonic change in the opticalabsorption coefficient as a function of temperature. In the specificexample to which the invention is applied herein, the controllingphenomenon is the narrowing of the bandgap of the semiconductor [it isthe direct optical bandgap in the case of GaAs] with increasingtemperature. Since the absorption coefficient for light of a narrowspectral range, whose photon energy is slightly higher than the bandgapenergy, depends on the separation between these two energies (i.e. thephoton energy and the bandgap energy: see Eq. 2), it follows that theabsorption coefficient will depend on the temperature of the GaAs waferor thin layer. The energy of the narrow spectral range employed in thismode must be such that it stays above the band edge at all temperaturesof interest [if, at any temperature, the bandgap exceeds the spectralenergy, then the light will be transmitted unabsorbed and thus willcease to be a measure of the temperature].

Indeed, this invention is applicable to all materials whose opticalabsorption coefficient is a monotonic function of temperature. It isapplicable, in particular, to all semiconductors and is enhanced byselecting narrow optical spectral ranges very close to the respectivebandgaps. The underlying mechanism is the same as detailed in thisdescription of the invention as applied to GaAs: the absorption ofoptical radiation close to the bandgap [and exceeding the latter'senergy by a small amount] is a function of the bandgap. Since in allsemiconductors the bandgap is a function of temperature, the inventionapplies to all semiconductors. It further applies to semiconductorswherein the bandgap is either direct or indirect.

It is the object of the present invention to determine the exacttemperature of the semiconductor thin layer or wafer, without contactingit physically. The invention is based on measurement of opticaltransmission, utilizing a properly selected narrow band spectral range,which undergoes moderately weak absorption as it transits through thesemiconductor. That optical transmission depends on the bandgap of thesemiconductor medium. The bandgap, in turn, is a function of thetemperature of that semiconductor layer within which the absorptiontakes place. Consequently, the optical transmission depends on thetemperature of the layer or the wafer.

The present invention does not only provide a method of accuratedetermination of the temperature, but it furthermore is employed --through an electrical feedback loop -- to control said temperature byadjusting the power to the heating agent. In general, an independent andconstant light source can provide the light whose absorption in thewafer or layer is used to monitor the latter's temperature and,utilizing a loop to control the separate heating agent, maintain theabove temperature at any desired value or values.

In the specific application of the invention to "heat cleaning" thewafer as described here, the heating agent is an incandescent projectionlamp emitting white light which heats the wafer by being partly absorbedin it. The method of this invention is applied by selecting a verynarrow spectral range of the white light from the projection lamp, andmeasuring its absorption by the wafer. In other words, the heating ofthe wafer and the light whose absorption is measured to monitor thetemperature are both provided by the same source (the lamp).

Since the intensity of the lamp varies during the "heat-cleaning"process, the application of the invention contains a continuouscomparison of the intensities of the light component transmitted throughthe wafer, with that emitted by the lamp. This "normalization" procedureenables us to separate those changes in transmission through the waferwhich are due to the latter's varying temperature, from changes whichare due to variations in the light intensity emitted by the lamp.

This invention overcomes all the aforementioned shortcomings of existingpyrometric methods.

These and other objects of the invention are achieved according to theinvention by providing a source of optical radiation having a desiredspectral component and directing that optical radiation to a layer ofmaterial having a bandgap which varies as a function of temperature. Theoptical radiation transmitted through the layer of semiconductormaterial is detected and analyzed to determine the optical absorptionwhich has occurred. Due to the relationship between direct bandgap andoptical absorption, analysis of the transmitted optical radiation willprovide an indication of the direct bandgap of the material which, inturn, is indicative of the material's temperature.

For a semiconductor wafer or layer, an in situ temperature determinationmay be accomplished while the wafer is in a heating chamber even thoughthe temperature detection apparatus is maintained outside the heatingchamber. Of course, the temperature detection apparatus could just aswell be wholly or partially within the chamber if it is tolerant of theprocessing temperatures. A light source, which may double as a heatsource, is provided which emits light including light within a spectralrange having a photon energy slightly higher than the bandgap energy ofthe semiconductor. Since the absorption coefficient for this spectralrange depends on the separation between the photon energy and thebandgap energy, it is possible to derive information relating to thebandgap by examining the absorption by the GaAs wafer in the spectralrange of interest. Additionally, the direct bandgap of GaAs narrows astemperature increases. Thus, information regarding the temperature ofthe GaAs wafer may be derived from the absorption of the identifiedspectral range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a layer of GaAs on a supporting structure which issuitable for temperature monitoring in accordance with the invention.

FIG. 2 is a plot showing the direct bandgap and optical transmission ofGaAs as a function of temperature.

FIG. 3 illustrates a calibration setup for the invention.

FIG. 4 is a plot showing normalized optical transmission duringcalibration.

FIG. 5 illustrates a setup for monitoring the sample's temperature byimplementing the invention.

FIG. 6 is a plot showing normalized optical transmission in accordancewith the invention and the temperature of the GaAs layer, during a"typical" heat-cleaning run.

FIG. 7 is a schematic of a prototype of the invention.

FIG. 8 is a photograph, illustrating a preferred manner of practicingthe invention.

FIG. 9 is a schematic illustration of another preferred manner ofpracticing the invention.

FIG. 10 is a plot showing optical transmission of multiple wavelengthsas a function of the temperature of a GaAs layer [measurements performedas in FIG. 3].

DETAILED DESCRIPTION OF THE DRAWINGS

An example of a layer of GaAs whose temperature may be directlydetermined according to the invention is the GaAs layer 101 on thesurface of the GaAlAs layer 103 bonded, via the thin dielectric coating105 to a glass faceplate 107, as shown in FIG. 1. Note that the "window"layer 103 is GaAlAs, with a much wider bandgap than the GaAs activelayer 101. The property of the material on which this invention is basedis the dependence of the bandgap on temperature, and the dependence ofthe optical absorption coefficient of light of a given energy on thedifference between the latter and the bandgap energy (which must besmaller than the light energy). This property is found for instance inGaAs, other III-V compounds and other semiconductor materials such assilicon, CdS or HgCdTe. It applies to direct as well as indirectbandgap, although it is easier to exploit in the former. The aboverelationships may be expressed in the following manner:

    Eg=Eg(T)                                                   1.

    α(hν)=α(hν-Eg), where hν≧Eg    2.

    hence, α=α(T)                                  3.

Relation No. 1 states that the semiconductor bandgap energy, Eg, is afunction of the temperature, T. Relation No. 2 states that theabsorption coefficient, α, for light of energy hν (where h is Planck'sconstant and ν is the frequency) is a function of the difference betweenthat energy and the bandgap energy Eg. Relation No. 3 states that as aresult of the above, the absorption coefficient is a function oftemperature.

The pictorial rendition of the above is found in FIG. 2. The upper partof the figure shows the change in the direct bandgap of a semiconductoras a function of temperature (the example is a semiconductor, such asGaAs, whose direct bandgap decreases as the temperature increases). Itis noted that the bandgaps of all semiconductors, whether such bandgapsbe direct or indirect, are functions of temperature. In mostsemiconductors, the bandgaps narrow upon increasing the temperature: thevalues of dEg/dT range from -14×10⁻⁴ ev/°C. in selenium to -0.3×10⁻⁴ev/°C. in tellurium, and the values of III-V compound semiconductorsclustering between -3×10⁻⁴ ev/°C. and -4×10⁻⁴ ev/°C. In a fewsemiconductors, notably in the IV-VI compounds, the bandgaps widen uponincreasing the temperatures, with values near +4×10⁻⁴ ev/°C. An exampleof a totally different class of materials where this invention isapplicable, is a colloidal suspension of small particles in a matrix ofanother material, the more so if that matrix is amorphous, gel or aliquid ["Christiansen Filter"]. In such cases, the absorption inselected spectral ranges is a strong function of temperature, making thepresent invention very applicable as a means to detect as well ascontrol the temperature. The lower part of the figure shows theconcomitant shift in the optical transmission curve.

The device of this invention measures the optical transmission at aselected wavelength through the sample whose temperature is to bedetermined, and is based on the temperature dependence of the opticalcoefficient of absorption.

The optical transmission of a selected wavelength through a given sampledepends not only on its coefficient of absorption and on the thicknessof the sample, but also on the reflectance properties at the variousinterfaces, on the level of doping and on possible stresses. The effectsof these other factors on optical transmission through the sample arealmost independent of temperature.

The manner of calibrating the transmission measuring device isdemonstrated in FIG. 3. The light from a lamp 301 is chopped by achopper 303 and then split by a beamsplitter 305 into a "reference"channel 311 and a "signal" channel 313 which goes through the prototypesample 307. The two filters 302a, 302b in the respective channels arenarrow bandpass filters which select the operating light energy(wavelength). The lock-in amplifier 304b monitors the sample signal viasample detector 308b, and lock-in amplifier 304a reads the referencesignal received via reference detector 308a. The signals through thecathode, S, and the reference, R, are constantly compared to each other.The transmission is then simply the ratio of the former to the latter.The oven temperature is monitored by the thermocouple 306. The sample,in the oven enclosure, is at the temperature indicated by thethermocouple. A typical calibration curve is shown in FIG. 4, where thetransmission was taken to be unity at room temperature. The schemeillustrated in FIG. 3 embodies a generic approach to measure thesample's temperature by the transmission method, resulting in the curveof FIG. 4.

As mentioned earlier, a specific utilization of this device is tomonitor the temperature of a GaAs layer on a photocathode during "heatcleaning", or the temperature of a wafer (such as GaAs) during "heatcleaning" prior to growing epitaxial layers on the GaAs, for instance bythe Molecular Beam Epitaxy method. In both cases, the source of thelight used to measure transmission through the sample may convenientlybe the same source used to heat the sample. Consequently, the carefulmonitoring of the light-source intensity, which changes during theheat-cleaning cycle, is essential.

The use of the invention in "heat cleaning" the photocathode sample 503is shown in FIG. 5. The lamp 504 and the cathode 503 are shown withinthe vacuum chamber 510. With the cathode 503 out of the way in position501, the sample channel 505 and the reference channel 506 are comparedfor normalization purposes. The cathode 503 is then moved into the pathof the light at position 502, and a "room temperature reference signal"is taken, with the heating lamp at a sufficiently low intensity so asnot to heat the cathode during this step. Next, the "heat cleaning"cycle proceeds as the lamp intensity is increased.

The signals from the two channels are acquired by a computer, not shown,through two inputs of an A/D converter, not shown, and they areconstantly being compared to provide a normalized (to constant opticalflux) transmission profile which is converted to a temperature profileaccording to the calibration of FIG. 4. A typical "heat cleaning"process as monitored according to the invention is shown in FIG. 6. Thetransmission is seen to decrease as time goes on, due to the increase ofthe temperature of the cathode's active layer. The correspondingtemperature can be deduced from FIG. 4 and incorporated into thesoftware which is used to provide the plots similar to those shown inFIG. 6. By doing this, a time-dependent temperature profile can beentered into the software, and the transmission would act as athermometer which reads the real-time temperature and compares it (atfrequent intervals) to the above "dialed-in" temperature. In FIG. 6, asan example, are seen two time plots: the nominal faceplate temperatureread by the IRCON, and the cathode temperature indicated by thetransmission through the cathode [in FIG. 6, the transmission valueswere not converted to cathode temperature; this would be done by usingdata at FIG. 4].

A description of the computer program to monitor the heat cleaningtemperature and to control it, follows.

The software to control the heat clean process is written in a versionof BASIC designed for measurement and control purposes. The program usesthe multitasking capability of this language to measure and control theprocess and to provide screen display, data storage and data printout.

With the cathode in the heat clean position, the heat clean lamp isoperated briefly at low wattage. During this time, sampling measurementsare made over a narrow wavelength band of light flux through the cathodeand of light flux from the lamp. From these measurements a "roomtemperature" normalization ratio is computed. Subsequent measurements oftransmission through the cathode are divided by this number to normalizethe transmission to the room temperature value.

After a normalization ratio has been calculated, the heat clean processis begun. The computer periodically samples light flux from the lamp andthrough the cathode and calculates the transmission relative to the roomtemperature transmission. This value is compared to a setpoint valuedetermined from a specified transmission versus time profile. Theprogram is structured to readily accommodate changes in the desiredshape and complexity of this curve. Using a PID(proportional-integral-derivative) algorithm, a correction to the lampwattage is calculated and a corresponding command voltage is sent to thepower supply controlling the lamp. This sequence is periodicallyrepeated until the heat clean process is completed.

A preferred manner of practicing the invention utilizes an arrangementwhere a single-lens camera 711 is used to focus the lamp filament 713,viewed through the cathode 715, onto the detector surface 717 which isin the camera's focal plane. This is shown schematically in FIG. 7. Thedevice itself, embodying the reference and sample channels, is shown inFIG. 8.

An expanded version of the invention is illustrated in FIG. 9. Here, thetemperature of the sample 91 is monitored by measuring the opticaltransmission properties of a group of wavelengths (all corresponding tooptical energies slightly above the bandgap). The source of opticalradiation 90 is a heat clean bulb which is aligned with a plurality ofslits 92a, 92b arranged to substantially collimate the light The sample91, which comprises a layer 901 of a material having a direct bandgapwhich varies as a function of temperature, GaAs in this example, isplaced so as to intercept the light between the bulb 90 and the slits92. Thus, the light passing through the slits 92 has traversed the layer901 of GaAs. A diffraction grating 93, a holographic, blazed grating inthis example, is placed at an angle to the collimated light beam exitingslits 92 such that the light is dispersed, in a dispersed beam 94, ontothe optical detector 95 which in this example is a linear detectorarray. The low frequency cutoff 96 is detected by the optical detectorthus permitting temperature detection according to the proceduresdescribed with respect to the prior examples. Additional benefits can beobtained by the use of a detector capable of isolating a plurality ofdiscrete wavelengths of optical energy. Since the functional dependenceof α on (h ν-Eg) [see Eq. 2] depends on the magnitude of (h ν-Eg) [itapproaches the asymptotic form α=constant x (h ν-Eg)^(1/2) ], such anarray would optimize the sensitivity of the method in differenttemperature ranges and will enhance the precision (which depends onnormalization at room temperature).

A calibration run at four wavelengths near the band edge is shown inFIG. 10.

This method, and the device, are applicable to measure the temperaturesof any semiconductor wafers or thin layers wherein the bandgap, andtherefore the transmission, is temperature dependent.

In a layered structure, the method is usually limited to that layer withthe narrowest bandgap.

The method requires a light source emitting an optical component whichis only partly absorbed in the sample. The method is particularlyapplicable to monitoring the temperature, in the manner indicated, inthose processes wherein the optical source acts as the heating source.

The method not only monitors and controls the temperature during theheating and "high temperature soak" (at a preset temperature) part ofthe cycle (see FIG. 6), but it also monitors the cooling rate of thesample and determines when the sample has cooled down to a presettemperature.

It is suited for any and all applications where physical contacts to thesample are undesirable, yet it could be applied equally well tosituations where the heating mechanism is non-optical.

We claim:
 1. A method employed in a system for detection of thetemperature of a bandgap material having a temperature dependent bandgapin a given energy range comprising the steps of:providing optical energyhaving a photon energy slightly higher than the top of said given energyrange such that said optical energy passes through said bandgapmaterial; receiving said optical energy after passage through saidbandgap material and as a function of the absorption by said bandgapmaterial of said optical energy, providing an output indicative of thetemperature of said bandgap material.
 2. The method according to claim 1wherein said step of providing optical energy includes the provision ofthermal radiation operative to heat said bandgap material.
 3. The methodaccording to claim 1 wherein said step of providing optical energyincludes the provision of optical energy having a color spectrum;andwherein said step of receiving said optical energy employs a detectorarray located to receive optical energy reflected from a holographicblazed grating.
 4. The method according to claim 1 wherein said step ofproviding an output as a function of the absorption by said bandgapmaterial of said optical energy includes the additional stepsof:providing a portion of said optical energy having a photon energyslightly higher than the top of said given energy range in a directionsuch that said portion of said optical energy does not reach saidbandgap material; and receiving said portion of said optical energy andgenerating a reference signal for use in determining the extent ofabsorption by said bandgap material of said optical energy.
 5. A methodof indicating the temperature of a layer of bandgap material having atemperature dependent bandgap in a given energy range comprising thesteps of:passing optical energy through said layer, said optical energyhaving a photon energy slightly higher than the top of said given energyrange. detecting the energy level of light absorbed by said layer andgenerating a signal indicative of the temperature of said layer.